The star formation region NGC 6530: Distance, ages and initial mass function
We present astrometry and BVI photometry, down to V≃22, of thevery young open cluster NGC 6530, obtained from observations taken withthe Wide Field Imager camera at the MPG/ESO 2.2 m Telescope. Both the Vvs. B-V and the V vs. V-I color-magnitude diagrams (CMD) show that theupper main sequence is dominated by very bright cluster stars, while,because of the high obscuration of the giant molecular cloud surroundingthe cluster, the blue envelopes of the diagrams at V≳14 are limitedto the main sequence stars at the distance of NGC 6530. This particularstructure of the NGC 6530 CMD allows us to conclude that its distance isabout d ≃ 1250 pc, significantly lower than the previousdetermination of d=1800 pc. We have positionally matched our opticalcatalog with the list of X-ray sources found in a Chandra-ACISobservation, finding a total of 828 common stars, 90% of which arepre-main sequence stars in NGC 6530. Using evolutionary tracks of\citet{sies00}, mass and age values are inferred for these stars. Themedian age of the cluster is about 2.3 Myr; in the mass range (0.6-4.0)Mȯ, the Initial Mass Function (IMF) shows a power lawindex x=1.22±0.17, consistent with both the Salpeter index(1.35), and with the index derived for other young clusters; towardssmaller masses the IMF shows a peak and then it starts to decrease.Based on observations made with the European Southern Observatorytelescopes obtained from the ESO/ST-ECF Science Archive Facility.Full Tables 4 and 5 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/430/941
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Anomalous dust-to-gas ratios in the Galaxy
Lines of sight with E(B-V)/N(HI) considerably smaller than the averagevalue for the solar neighbourhood have been selected from the catalogueof Diplas & Savage. In order to develop quantitative considerations,estimates of the molecular hydrogen column density were obtained usingthe relation of Savage et al. extended at E(B-V) > 0.4 with therecent data of Rachford et al. Contrary to the prevailing opinion in theliterature for sightlines with similar behaviour, we found that only 22per cent of our sample was characterized by both an average gas densitylarger than 1 cm-3 and a value of RV larger thanthat in the diffuse interstellar medium. By computing extinction models,we were able to fit the E(B-V)/N(HI) by changing the value ofRV only for some sightlines. For the remaining ones, aρd/ρH ratio different from the averageGalactic value must be invoked. The application of the Kramers-Kronigrelation to the observed extinction curves confirmed this possibility.Moreover, attempts to fit such curves with models having grain volumescorresponding to the standard ρd/ρH ratiofailed.We find a linear relation between ρd/ρHand E(B-V)/N(H) for our sightlines. The average Galactic value marks theseparation into two groups characterized by lower abundances of C and Sitrapped into the grains when E(B-V)/N(H) is smaller than the Galacticvalue, and by larger abundances when E(B-V)/N(H) is greater.
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A Deep Chandra X-Ray Observation of the Rich Young Cluster NGC 6530. I. The X-Ray Source Catalog and the Cluster Population
In a deep 60 ks Chandra ACIS X-ray observation of the very young clusterNGC 6530, we detect 884 X-ray point sources and argue that a very largefraction of them (90%-95%) must be pre-main-sequence (PMS) clustermembers, mostly of low masses. This is a significant enlargement of theknown NGC 6530 stellar population with respect to previous opticalstudies, including Hα surveys. We identify 220 X-ray sources withcataloged stars down to V=17, while most unidentified sources havefainter counterparts. Moreover, we find an infrared counterpart in the2MASS catalog for 731 X-ray sources. The optically identified clusterX-ray sources are found in a band in the H-R diagram above the mainsequence, in the locus of 0.5-1.5 Myr PMS stars, with masses down to0.5-1.5 Msolar. We find evidence of an age gradient acrossthe field from northwest to south, suggesting a sequence of starformation events qualitatively similar to that found in earlier studiesof the same region, but differing in the details. A group of X-raysources showing frequent flares may be associated with the youngeststars in the cluster, suggesting that X-ray flaring activity isespecially intense in the youngest PMS phases of low-mass stars.
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Absolute proper motions of open clusters. I. Observational data
Mean proper motions and parallaxes of 205 open clusters were determinedfrom their member stars found in the Hipparcos Catalogue. 360 clusterswere searched for possible members, excluding nearby clusters withdistances D < 200 pc. Members were selected using ground basedinformation (photometry, radial velocity, proper motion, distance fromthe cluster centre) and information provided by Hipparcos (propermotion, parallax). Altogether 630 certain and 100 possible members werefound. A comparison of the Hipparcos parallaxes with photometricdistances of open clusters shows good agreement. The Hipparcos dataconfirm or reject the membership of several Cepheids in the studiedclusters. Tables 1 and 2 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
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UBVRI and Hα Photometry of the Young Open Cluster NGC 6530
New UBVRI and Hα photometry has been obtained for the young opencluster NGC 6530 located 10' east of the Lagoon Nebula (M8). We found 37pre-main-sequence (PMS) stars with Hα emission and nine PMScandidates using the R-Hα color, a measure of Hα emission.From this new photometry we derived the cluster parameters(V0-MV=11.25+/-0.1 mag and =0.35mag), typical age (1.5 Myr), and an age spread of about 5 Myr. Weconfirmed the presence of a small amount of differential reddeningacross the cluster. We also verified the abnormal reddening law for thecluster, especially for a heavily embedded O7 V star, Herschel 36.Comparisons were made for several PMS evolution models with regard tothe resulting age distribution and the initial mass function.
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UBV beta Database for Case-Hamburg Northern and Southern Luminous Stars
A database of photoelectric UBV beta photometry for stars listed in theCase-Hamburg northern and southern Milky Way luminous stars surveys hasbeen compiled from the original research literature. Consisting of over16,000 observations of some 7300 stars from over 500 sources, thisdatabase constitutes the most complete compilation of such photometryavailable for intrinsically luminous stars around the Galactic plane.Over 5000 stars listed in the Case-Hamburg surveys still lackfundamental photometric data.
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A multiwavelength study of star formation in the very young open cluster NGC 6530
The distance and formation history of the very young open cluster NGC6530 were investigated by studying a total of 132 probable member starsof this open cluster. It was concluded that the distance to NGC 6530 is1.8 +/- 0.2 kpc, and its interstellar reddening E(B-V) is 0\fm30.Furthermore, we conclude that the extinction law of the intraclustermaterial is normal (i.e. R_V = 3.1), but anomalous extinction laws werefound for several more embedded stars in the cluster. Of the 132 starsincluded in this study, 11 are suspected to be variable, five showHα in emission and nine show an infrared excess. Among the memberstars of NGC 6530, three were found to be part of the Herbig Ae/Bestellar class, whereas two others are possible members of this stellargroup as well. Also, one cluster member is probably a new massivepost-AGB star, whereas the same could possibly apply to another memberstar. Finally, from the distribution of post- and pre-main sequencestars in the cluster's HR-diagram, it was concluded that the process ofstar formation in NGC 6530 must have started a few times 10^7 years agoand, for the less massive stars, is probably still going on today. Basedon observations collected at the European Southern Observatory, LaSilla, Chile, on observations collected at Cerro Tololo Inter-AmericanObservatory, and on spectral data obtained with the InternationalUltraviolet Explorer. Tables 1, 2 and 5 are available in electronic format the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia http://cdsweb.u-strasbg.fr/Abstract.html
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A Radial Velocity Database for Stephenson-Sanduleak Southern Luminous Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1997AJ....113..823R&db_key=AST
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Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue.
We give a common version of the two catalogues of Mean Radial Velocitiesby Wilson (1963) and Evans (1978) to which we have added the catalogueof spectroscopic binary systems (Batten et al. 1989). For each star,when possible, we give: 1) an acronym to enter SIMBAD (Set ofIdentifications Measurements and Bibliography for Astronomical Data) ofthe CDS (Centre de Donnees Astronomiques de Strasbourg). 2) the numberHIC of the HIPPARCOS catalogue (Turon 1992). 3) the CCDM number(Catalogue des Composantes des etoiles Doubles et Multiples) byDommanget & Nys (1994). For the cluster stars, a precise study hasbeen done, on the identificator numbers. Numerous remarks point out theproblems we have had to deal with.
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The Henry Draper Extension Charts: A catalogue of accurate positions, proper motions, magnitudes and spectral types of 86933 stars
The Henry Draper Extension Charts (HDEC), published in the form offinding charts, provide spectral classification for some 87000 starsmostly between 10th and 11th magnitude. This data, being highlyvaluable, as yet was practically unusable for modern computer-basedastronomy. An earlier pilot project (Roeser et al. 1991) demonstrated apossibility to convert this into a star catalogue, using measurements ofcartesian coordinates of stars on the charts and positions of theAstrographic Catalogue (AC) for subsequent identification. We presenthere a final HDEC catalogue comprising accurate positions, propermotions, magnitudes and spectral classes for 86933 stars of the HenryDraper Extension Charts.
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A spectroscopic database for Stephenson-Sanduleak Southern Luminous Stars
A database of published spectral classifications for objects in theStepenson-Sanduleak Luminous Stars in the Southern Milky Way catalog hasbeen compiled from the literature. A total of 6182 classifications for2562 stars from 139 sources are incorporated.
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An IUE survey of interstellar H I LY alpha absorption. 1: Column densities
We measure Galactic interstellar neutral hydrogen column densities byanalyzing archival interstellar Ly alpha absorption line data toward 554B2 and hotter stars observed at high resolution with the IUE satellite.This study more than doubles the number of lines of sight with measuresof N(H I) based on Ly alpha. We have included the scattered lightbackground correction algorithm of Bianchi and Bohlin in our datareduction. We use the correlation between the Balmer discontinuity(c1) index and the stellar Ly alpha absorption in order toassess the effects of stellar Ly alpha contamination. Approximately 40%of the B stars with measured (c1) index, exhibit seriousstellar Ly alpha contamination. One table contains the derived values ofthe interstellar N(H I) for 393 stars with at most small amounts ofstellar contamination. Another lists the observed values of total N(H I)for 161 stars with suspected stellar Ly alpha contamination and/oruncertain stellar parameters.
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Ultraviolet gas absorption and dust extinction toward M8
Interstellar absorption lines are analyzed using high-resolution IUEspectra of 11 stars in the young cluster NGC 6530 located in the M8region. High-velocity clouds at -35 km/s and -60 km/s are seen towardall cluster stars. The components arise in gases that are part of largeinterstellar bubbles centered on the cluster and driven by stellar windsof the most luminous members. Absorption lines of species of differentionization states are separated in velocity. The velocity stratificationis best explained as a 'champagne' flow of ionized gas away from thecluster. The C IV/Si IV ratios toward the hotter cluster members areconsistent with simple photoionization models if the gas-phase C/Siratio is increased by preferential accretion onto dust grains. High ioncolumn densities in the central cluster decline with distance from W93,suggesting that radiation from a hot source near W93 has photoionizedgas in the central cluster.
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Getting to the bottom of the lagoon - Dust, magnetism, and star formation
A V-band polarimetric survey of stars associated with the Lagoon Nebula(M8; NGC 6523) and the embedded cluster NGC 6530 has been conducted. Thedata have been combined with existing photometric and spectroscopicobservations in order to study the dust in the immediate vicinity and toinvestigate the alignment of magnetic field lines with identifiablesymmetry axes. Using the cluster method, it has been possible toseparate multiple stars from single stars and thereby derive acccuratevalues for the ratio of total to selective extinction and the distancemodulus. The value of R for the associated dust is 4.6 + or - 0.3.Anomalous reddening is a characteristic of the region, not just Herschel36. Electric vectors show a high degree of alignment perpendicular tothe direction of the extension of the large-scale distributions ofionized gas and massive stars. The overall collapse of molecular gasappears to have been asymmetrical, proceeding most rapidly along adirection parallel to magnetic field lines. A lot of the dust mixed withthe ionized gas has been destroyed, so the large-scale uniformity ofreddening laws and polarization position angles indicates that much ofthe dust responsible for extinction and polarization is associated withneutral gas.
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Ultraviolet extinction differences within NGC 6530
Ultraviolet extinction curves are presented for 14 stars in the veryyoung cluster NGC 6530. The variation of the short-wavelength extinctionis consistent with that expected based on random errors alone, althoughthe weakest extinction is found only toward the cluster center.Variations significantly larger than the measurement uncertainty arefound in the area, width, and central peak position of the 2200 Aextinction bump. These parameters are largest in the cluster center anddecrease systematically outwards. Physical processes capable ofmodifying the grains in the H II region containing the cluster arediscussed.
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Ultraviolet interstellar extinction toward NGC 6530 and the intrinsic energy distribution of 9 Sagittarii and HD 165052
The ultraviolet interstellar extinction curve is determined toward 12early B main-sequence stars and one O8 V star in NGC 6530. Theextinction is found to be uniform across the cluster. At shortwavelengths, the extinction curve of NGC 6530 falls below the averageGalactic curve by a full magnitude, indicating that the population ofsmall grains relative to larger grains is lower than normal. Longward of2830 A the curve is almost coincident with the mean Galactic curve. The2175 A feature is narrower than the average. The derived extinctioncurve is used to deredden the observed fluxes of two early O-type starsin NGC 6530:9 Sgr and HD 165052. The intrinsic continua are compared tomodel atmospheres in the region 1200-8200 A. The LTE, line-blanketedmodels of Kurucz (1979) fit the observations better than the non-LTE,unblanketed models of Mihalas (1972) suggesting that line-blanketingeffects are more important than the non-LTE effects in the atmospheresof these stars. The temperatures of the models that best fit theobservations are lower than the Zanstra temperatures and thetemperatures estimated from the strengths of the He I and He II lines,by 10,000 to 20,000 K.
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UBV and H-beta observations of stars towards M8
New and more accurate UBV magnitudes have been obtained for 106 starswith known proper motions in the field of the emission nebular M8. Thecolor-magnitude diagram of the embedded cluster NGC 6530 consists of amain sequence from B1 to about B9 with a distance modulus of 11.4 mag.Spectral types earlier than B1 and later than B9 fall above the mainsequence. Evidence is given that the so-called pre-main sequence of NGC6530 is not related to this cluster but consists of field stars. H-betaobservations strongly support previous results that many cluster membersare intrinsic Be stars.
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Study of the extremely young cluster NGC 6530.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1978MNRAS.184..467S&db_key=AST
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Broad-band photometry of NGC 6530.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1977MNRAS.178..423K&db_key=AST
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A study of Be stars in clusters.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1976ApJ...204..493S&db_key=AST
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Membership in the extremely young open cluster NGC 6530(M8).
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1972A&A....20..425V&db_key=AST
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Studies in Spectral Classification. II. The HR Diagram of NGC 6530.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1965ApJ...141..183H&db_key=AST
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Correction of background effect in photographic photometry.
Not Available
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A Catalogue of H II Regions.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1959ApJS....4..257S&db_key=AST
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Studies of Extremely Young Clusters. II. NGC 6530.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1957ApJ...125..636W&db_key=AST
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A Catalogue of Emission Nebulae Near the Galactic Plane.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1953ApJ...118..362S&db_key=AST
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INHABITED SKY
